1. Field of the Invention
The present invention relates to a method of performing a data-recording and/or data-reading operation with respect to an optical storage disk. In this specification, an xe2x80x9coptical storage diskxe2x80x9d may refer to, unless otherwise specified, any type of data storage medium with which desired information is written or read optically. For instance, the optical storage medium may be a read-only disk, magneto-optical disk or phase change optical disk.
2. Description of the Related Art
One way to improve the data storage capacity of an optical disk is to increase the data recording density of the disk. A higher storage density may be achieved by making smaller the laser spot formed on the optical disk in recording data or reading data. Typically, the laser beam emitted from a light source is passed through an objective lens unit before it strikes upon the recording layer of the storage disk. As passing through the lens unit, the laser beam converges to make a small light spot on the storage disk. The storage disk may include a transparent substrate upon which a recording layer is formed. The laser beam emitted from a light source may be arranged to pass through the transparent substrate and then strike upon the recording layer. As known in the art, the diameter of the light spot increases in proportion to xcexc/NA, where xcexc is the wavelength of the laser beam and NA is the numerical aperture of the objective lens. As seen from this formula, the wavelength of the laser beam should be shorter, or the NA of the lens unit should be higher (or both) in order to make smaller the light spot.
However, the reduction of the wavelength (shorter than 400 nm for example) increases the laser beam energy. This is disadvantageous because the properties of the optical components incorporated in the disk apparatus may be damaged due to the high energy. In addition, a laser beam of a shorter wavelength is readily absorbed as passing through the transparent substrate of the storage disk. As a result, the reflection light of a shorter wavelength may fail to provide appropriately strong information signals. In light of these, a laser beam whose wavelength is shorter than 400 nm is not suitable for size reduction of the light spot.
When the NA increases, on the other hand, the data-processing operation may suffer large spherical aberration. As known in the art, the spherical aberration caused by the thickness error of the disk substrate increases in proportion to the thickness error with a factor of the 4th power of the NA of the lens. Unfavorably, the occurrence of spherical aberration produces an out-of-focus laser spot, as in the case where the objective lens is xe2x80x9cdefocused.xe2x80x9d (In this specification, xe2x80x9cdefocusxe2x80x9d is used for describing the positional deviation of the objective lens from a prescribed point along the optical axis of the lens.) Therefore, when a high NA is desired, it is necessary to prevent spherical aberration from occurring due to the thickness error of the disk substrate.
JP-A-2000-21014 teaches that an objective lens is moved along the optical axis of the lens to cope with both the aberration caused by the defocusing of the lens and the aberration caused by the thickness error of the disk substrate. According to the disclosed method, a compensation signal to move the objective lens is generated based on the detection of interference rays contained in the reflection light from the storage disk. The disclosed light interference occurs by the interaction of 0-order diffracted rays (whose incident angle is equal to its reflection angle) with 1-order diffracted rays (whose incident angle is smaller than its reflection angle).
Referring to FIG. 8 of the accompanying drawings, when the data-storing lands L of the disk D are irradiated by a laser beam, 0-order diffracted rays Di0 and 1-order diffracted rays Di1 will appear. As illustrated in the figure, these diffracted rays partially interfere with each other (see the hatched area F). When the substrate thickness of the storage disk D is normal, the intensity of the interference light (as measured in the hatched area F) exhibits no variation, as shown in FIG. 10A. When the substrate thickness is not normal (namely, unduly small or large), the light intensity in the hatched areas F is not uniform (see FIGS. 10B and 10C), and spherical aberration occurs.
The same problem is encountered when the objective lens is defocused. Specifically, when the objective lens is deviated negatively (that is, the distance between the objective lens and the disk D is too small), a pattern as shown in FIG. 10B is observed. When the lens is deviated positively (i.e., the distance between the lens and the disk D is too large), a pattern as shown in FIG. 10C is observed. As seen from the graph in FIG. 8 (the graph shows light intensity variations measured along the single-dot chain line DL), the light intensity varies in a similar manner for both a case where the objective lens is positively defocused and a case where the disk substrate has an unduly large thickness. Though not depicted, the light intensity curve for the negative lens defocus exhibits a similar variation to that of the light intensity curve for the disk substrate having an unduly small thickness.
The circular figure LA (the lowest illustration) in FIG. 8 shows the layout of light-receiving sections of an optical detector disclosed in the above JP document (2000-21014). Based on the detected intensity of interference light, the detector LA generates compensation signals for the defocus of the lens and for the substrate thickness error. The detector LA is provided with two light-receiving areas La each of which is divided into two sections A and B for detection of interference light in the hatched areas F. The section A is responsible for receiving light from the inner half (i.e., the half closer to the center of the 0-order diffracted ray Di0) of the hatched area F, while the section B is responsible for receiving light from the outer half (i.e., the other half farther from the center of the 0-order diffracted ray Di0) of the hatched area F. The compensation signal generated by the detector LA corresponds to the difference between the light intensity IA detected by the section A and the light intensity IB detected by the section B. It should be noted here that the prior art compensation signal is generated without distinguishing the light intensity variation stemming from the lens defocusing and the light intensity variation stemming from the thickness error of the disk substrate. When the disk substrate has an unduly large thickness or the objective lens is positively defocused, the light intensity in the hatched area F becomes greater in an outer part of the area F. Thus, in this case, the compensation signal becomes negative (xe2x88x92). On the other hand, when the disk substrate has an unduly small thickness or the objective lens is negatively defocused, the compensation signal becomes positive (+). In accordance with the polarity and magnitude of the compensation signal, the objective lens may be moved along the optical axis toward or away from the storage disk, so that the compensation for the lens defocus and the substrate thickness error is made.
In the above method, the objective lens is moved along its optical axis for making the compensation. In this manner, while the intensity variation caused by the defocusing of the objective lens can be corrected (see FIG. 9A), the intensity variation caused by the thickness error of the disk substrate may remain uncorrected (see FIG. 9B). This difference results from the non-negligible discrepancy between the intensity curve stemming from the lens defocusing (see the graph in FIG. 8) and the intensity curve stemming from the substrate thickness error. Specifically, the aberration due to the thickness error increases in proportion to the thickness variation with a factor of the 4th power of the NA, whereas the aberration due to the lens defocusing increases in proportion to the lens deviation with a factor of the 2nd power of the NA. Accordingly, the unfavorable variation of the light intensity cannot be completely eliminated. Thus, conventionally, the peak intensity of the laser spot on the storage disk fails to be sufficiently high.
The fail in providing appropriate peak intensity is disadvantageous in using a magneto-optical disk designed for implementing magnetically induced super resolution (MSR). Specifically, in an MSR disk, reproduction of the stored data is performed only at a high-temperature portion at the center of the laser spot formed on the disk. Thus, when the peak intensity is below the normally required level, the data reproduction from an MSR disk cannot be performed.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a method for overcoming the conventional drawbacks due to the thickness error of the substrate of a storage disk.
According to the present invention, there is provided a method of performing data-writing/data-reading with an optical data storage medium including a substrate. The method includes the steps of: irradiating the storage medium with light from a light source via an objective lens to make a light spot on the storage medium; moving the lens from a focus point to a defocus point where the light spot has a maximum peak intensity; and detecting the defocus point as a minimum depletion point regarding aberration caused by a thickness error of the substrate.
In the above method, when some thickness error is observed in the substrate of the data storage medium, it is not the focus point of the lens but a particular defocus point (where the peak intensity of the light spot is maximized) that is designated as the minimum depletion point for the irradiation light. In this manner, it is possible to minimize the peak intensity reduction in the light spot that is caused by the thickness error of the substrate.
Preferably, the irradiation may produce a 0-order diffracted ray and a 1-order diffracted ray, wherein the 0-order diffracted ray and the 1-order diffracted ray interfere with each other to generate interference light. Based on this interference light, the detection of the minimum depletion point may be performed. Since the interference light exhibits characteristic light intensity patterns, the minimum depletion point can be properly detected.
Preferably, the interference light may be detected by an optical detector provided with a light-receiving region divided into a plurality of sections.
Preferably, the plurality of sections may detect a thickness error signal regarding the substrate in accordance with a first detection format, while also detecting a defocus signal regarding the lens in accordance with a second detection format different from the first detection format.
Preferably, the detection of the minimum depletion point may be performed based on the thickness error signal and the defocus signal.
Preferably, the method of the present invention may further include the step of predetermining a relation between the thickness error signal and the peak intensity, wherein when the lens is at the minimum depletion point, the peak intensity of the light spot is calculated based on the predetermined relation.
Preferably, the method of the present invention may further include the step of adjusting an output of the light source based on a difference between a peak intensity of the light spot detected when the substrate has a normal thickness and a peak intensity of the light spot detected when the lens is at the minimum depletion point.
With the above arrangement, the temperature in the center of the light spot can be sufficiently high. Accordingly, even the data-reading or data-writing with respect to an MSR medium can be properly performed.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.